1. Field of the Invention
The present invention relates to a purge control apparatus for controlling the purging of hydrogen from a fuel cell stack.
2. Description of the Related Art
As a type of fuel cell stack to be installed in a fuel cell powered vehicle, a fuel cell stack is known which includes a fuel cell unit having an anode and a cathode that together sandwich a solid polymer electrolyte membrane therebetween, and in which a fuel gas (e.g., hydrogen gas) is supplied to the anode, and an oxidizing gas (e.g., oxygen or air) is supplied to the cathode so that electrochemical energy produced in an oxidation and reduction reaction of these reaction gases is directly output as electrical energy.
In general, in this type of fuel cell stack, unreacted hydrogen that it is purged from the fuel cell stack is collected and is mixed with fresh fuel gas, and then the gas mixture is supplied to the fuel cell stack again in order to increase fuel-use ratio and to improve fuel economy.
In addition, in this type of fuel cell stack, the reaction gases (hydrogen gas and/or oxidizing gas) are humidified in order to prevent the solid polymer electrolyte membrane from becoming dry so that power generation conditions can be maintained. Furthermore, in this type of fuel cell stack, water is produced by the power generating reaction. The produced water may remain at the anode side of the fuel cell stack (this condition is referred to as “flooding”), and the produced water may inhibit supply of the hydrogen gas, which may lead to unstable power generating conditions.
Furthermore, because nitrogen is contained in air that is supplied to the cathode, although only small amounts thereof permeate the solid polymer electrolyte membrane and are mixed with the hydrogen gas at the anode side, concentration of nitrogen may rise in the course of recycling the hydrogen gas, which may also lead to unstable power generating conditions.
Therefore, in order to ensure continuous stable conditions for power generation in a fuel cell stack, it is necessary to discharge water remaining at the anode side as well as nitrogen that is contained in the hydrogen gas. In order to discharge water and nitrogen, a purging operation is carried out in a circulating flow path for the hydrogen gas; however, the hydrogen gas, which should not be released to the atmosphere without being treated, is discharged as well at the same time in such a purging operation. One of the countermeasures for this problem is to mix the purged hydrogen with air discharged from the cathode side so that hydrogen at a low concentration is released to the atmosphere. A device used for reducing hydrogen concentration is referred to as a purged hydrogen dilution device.
However, designs of such purged hydrogen dilution devices have limitations in terms of installation space. In particular, in a fuel cell system to be installed in a fuel cell powered vehicle, installation space is even further constrained. Due to such limitations, purged hydrogen may not be sufficiently diluted in certain operating states of a fuel cell system.